Hoover et al., U.S. Pat. No. 2,958,593 describes and claims a low-density abrasive article comprising a uniform, lofty, open, non-woven, three-dimensional integral structure formed of many interlaced, randomly extended, flexible, durable, tough, resilient, organic fibers. The structure is unified throughout by firmly bonding fibers together at points where they cross and contact. Abrasive particles are distributed within the structure and firmly bonded to the fibers by an adhesive, which may perform the dual function of bonding web fibers together and bonding abrasive grains thereto. Alternatively, a first adhesive may be used for bonding the fibers to each other and a second adhesive for bonding the abrasive granules to the fibers. Despite great utility and broad commercial acceptance, most low density abrasive products heretofore known are subject to tearing when the abrasive is snagged on a burr or other projection, largely because of the hard, brittle nature of the phenolic resins generally used in making low density abrasives. Plasticizing a phenolic resin binder will lower its hardness and improve the tear resistance of a low density product made therewith, but as tear resistance increases, abrading effectiveness tends to decrease. Further, the phenolic systems not only have a tendency to degrade and weaken nylon fibers, but also show poor adhesion to polyester fibers, thereby limiting the potential of low density abrasives made from said phenolic systems using these resins.
Low density abrasive webs bonded with epoxy resins have excellent tear resistance, but commonly used, the cured resins do not have adequate heat-resistance for severe abrasive application. In addition, catalyzed epoxy resins often have an excessively short pot life, making it commercially difficult to bond the fibers of a low density abrasive with an epoxy resin. Epoxy novolac systems as disclosed and claimed in U.S. Pat. No. 3,510,283 have lower toxicity and better durability than other epoxy systems, but low density abrasive products bonded with the epoxy-novolacs tend to deposit melted fibers (known as smear) on the workpiece when exposed to the high stresses common in abrasive uses.
Another type of low density abrasive utilizes a flexible elastomeric binder, such as latex or a reclaimed rubber dispersion. The use of such binders gives a product which is useful in making scouring pads for hand use, but does not result in a low density abrasive useful in a wide variety of industrial applications where a hard binder is necessary to prevent smearing and maintain sufficient cut.
Prior to the present invention, the men skilled in the low density abrasive art faced a dilemma - either their products abraded well without smearing but tended to snag and tear, or they resisted tearing but abraded poorly and/or smeared.